Continuous process for producing magnesium metal from magnesium chloride including fused bath electrolysis



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FITTORNEH'S Dec 24, 1968 F. E. LOVE CONTINUOUS PROCESS FOR PRODUCING MAGNESIUM METAL FRO" IAGNESIUH CHLORIDE INCLUDING FUSED BATH ELECTROLYSIS 4 Sheets-Sheet Filed June 8, 1966 INVENTOR E. Low:

. ATTORNEYS United States Patent Ofifice 3,418,223 Patented Dec. 24, 1968 3,418,223 CONTINUOUS PROCESS FOR PRODUCING MAG- NESIUM METAL FROM MAGNESIUM CHLORIDE INCLUDING FUSED BATH ELECTROLYSIS Frank E. Love, Henderson, Nev., assignor to National Lead Company, New York, N.Y., a corporation of New Jersey Filed June 8, 1966, Ser. No. 556,108 7 Claims. (Cl. 204-70) This invention pertains to a continuous process for large-scale, quantity-production of high purity magnesium metal and chlorine by electrolysis of substantially anhydrous magnesium chloride.

The production of magnesium metal by electrolysis of magnesium chloride as presently practiced commercially, is generally carried out as a batch-type operation in a series of electrolytic cells, into each of which a mixture of metal chloride salts, such as the chloride salts of sodium, potassium, calcium and magnesium, in appropriate proportions, is initially charged, usually in a molten state, and subjected to electrolysis by passage of direct current between spaced anode and cathode electrodes dipping into the bath. The resulting electrolysis of the bath breaks dwon the magnesium chloride into magnesium metal and chlorine gas, which pass to the cathode and anode electrodes, respectively,for separate removal and recovery. The chlorine gas is continuously drawn off through ducts penetrating the cell covers, while the molten magnesium metal is manually taken off at intervals. Usually the salt bath employed is denser than the molten magnesium which accordingly rises to the bath surface and floats thereon for periodic removal as by manual ladling.

Accordingly production as thus commercially practiced involves operating a large number of individual cells which function independently of one another, each of which has different operating characteristics from the others and which must therefore be independently tended by workmen who must periodically take olf the accumulated magnesium metal and sludge from each cell. Also the melts must be individually analyzed and refurbished with magnesium chloride, and also occasionally with other of the salts comprising the bath make-up, to maintain the relative proportions of each within required operating limits. In addition, adjustments of the cell electrodes must be made to control the cell operating temperature. Operation of a large bank of such cells, therefore, involves high labor and maintenance, and requires rigid scheduling of charging and of metal and sludge removal, generally referred to as metalling and smutting. In addition the work on the cells thus required reduces the efliciency of operation and the quality of both the magnesium and chlorine products. Also where high humidity exists in the plant, the periodic opening of the cells for charging, metalling and smutting, decreases the cell efiiciency and increases the problems of handling the magnesium chloride in both the solid and molten states.

In accordance with the present invention these objectionable features of present commercial practices are eliminated, basically by a continuous process employing enclosed equipment wherein a large number of cells are operated as a unit in a continuous and automatic manner.

In the accompanying drawings:

FIG. 1 is a flow sheet illustrative of the process of the invention with the essential equipment components shown more or less diagrammatically; while FIG. 2 is an enlarged longitudinal sectional elevation, and FIG. 3 is a transverse section at 33 of FIG. 2, of one of the electrolysis cells of the FIG. 1 showing, and

illustrating the structural details and performance thereof under operating conditions.

FIGS. 4 and 5 are perspecitve views with parts removed to show the interior constructions of an apparatus for practicing the invention on a commercial scale, FIG. 5 being a continuaiton of the FIG. 4 apparatus.

Referring to FIG. 1, solid blocks substantially anhydrous magnesium chloride, such as cast or pressed blocks thereof, are fed upwardly over a conveyor 11, and introduced thence through a double-door assembly 12, into an enclosed preheating chamber 13, and delivered thence onto a downwardly-inclined chute 14a, along which the blocks 10 slide into a melt cell 15. An extension 14 of the preheating chamber enclosure houses the chute 14a and the opening into the melt cell 15 as shown. Into this housing is also fed HCl-containing hot gases over pipelines 16, 16a, collected from the electrolysis cell cathode compartments, launders, charging and refining cells, as hereinafter explained. These gases remove some of the surface moisture from the magnesium chloride and tend to reduce decomposition thereof.

The magnesium chloried blocks are melted in the melt cell 15, to form a molten bath 17 thereof, by electrical resistance heating provided by spaced electrodes, as at 18, 19, extending into the bath and energized from an alternating current suorce 20 for heating the salt without decomposition. In addition, spaced carbon electrodes, 21, 22, extend into the bath and are energized from a low voltage, direct current source of up to two volts or so for the purpose of decomposing oxygen-bearing compounds in the salt and converting the decomposed oxide to carbon monoxide gas evolved as such. The housing 14 is provided above the melt cell with an opening, closed by a removable cover 23, for sludge collection and removal, employing for such purposes a suitable device, such as a clam shell 24, back hoe, etc.

A feeder launder 25, extends from a level near the top of the melt cell into a charging cell 26, whereby the molten MgCl overflows from the melt cell into the charging cell. The launder is heated or suitably thermally insulated to prevent solidification of the molten salt in passing from the melt cell into the charging cell. Alternatively, the molten salt in the melt cell may be pumped or tapped intermittently over the launder 25 into the charging cell. The salt in the charging cell is heated by spaced electrodes 27, 28, energized from an A.C. source 29, to maintain the salt in molten condition in the charging cell.

The charging cell 26 receives over a conduit 30 from a refining cell 31, a circulating flow of cell melt cycled thereto from a series of electrolysis cells as hereinafter explained. This cell melt has been stripped in the refining cell of molten magnesium metal resulting from the electrolysis and also is deficient in magnesium chloride due to the electrolysis. The molten magnesium chloride from the melt cell 15 is added to this circulating cell melt at a controlled rate in the charging cell 26, and in such proportion as to maintain a specified magnesium chloride content in the melt of the charging cell. Conventional controls on the A.C. heating circuits for the melt and charging cells, control the temperature of the circulating melt and also the feed rate of solid magnesium chloride into the melt cell. I

The thus recharged cell melt, at controlled temperature, is pumped over a conduit 32 and into an enclosed and thermally insulated, inclined feeder launder 33 extending along and above a row of electrolysis cells, as at E -E inc. Thermally insulated conduits, as at P P inc., extend downwardly from the feeder launder 33 into the cells E E inc., respectively, and to a level below the surface level of the molten salt bath of each, for feeding the molten salt into these cells. Either intermittent or continuous feeding may be employed. Intermittent feeding has the advantages that a smaller quantity of circulating melt is required and there is elimination of the possibility of electrical shorting between the cells. Also the molten magnesium will be carried out of the cells more cleanly than otherwise, and by overflow therefrom as hereinafter described.

The cell melt is fed into the cells E E inc., under the surface of the molten bath therein via the conduits P P inc., and the displaced melt overflows out of the cathode compartments thereof together with the molten magnesium metal produced, into an overflow launder 34 via overflow conduits D -D inc., extending from the cells, respectively. The level of molten salt in each cell E -E inc., is maintained by a weir disposed in its associated overflow conduit, and, as semi-walls of the electrolysis cell fail, the cell level can be raised by adding bricks to the overflow weir, as explained below.

The overflow launder 34 is of opposite inclination to the feeder launder 33, and extends along the cells E E inc. The overflow from all electrolysis cells is thus collected in overflow launder 34, and conveyed thence over a conduit 35 into the refining cell 31. This cell is divided into two compartments by a deep semi-wall 36, so that the molten magnesium metal will float, as at 37, above the cell melt 38, in the first compartment thereof disposed on the right-hand side of the semi-wall 36. The cell melt 38 flows under the semi-Wall 36 and overflows to the charging cell 26 over the interconnecting conduit 30. A permanent stifl-flux cover 39 is maintained on the surface of the molten metal 37, and the refined molten magnesium metal is pumped intermittently to casting or alloying. The refining cell bath is electrically heated in the same manner as cells 15 and 26, but also can be cooled by a heat-exchanger (not shown) which normally will be perated to cool the molten magnesium metal to optimum casting temperature. The chlorine gas evolved from the electrolysis cells, is conveyed over conduits C C inc., extending from the respective cells to a manifold 40, which conveys the same to a liquefaction unit, or alternatively to a point for use as gaseous chlorine. Sludge collection and removal from cells 26 and 31 is accom plished in the same manner as for cell 15, as indicated by the dotted lines 41.

Referring now to FIGS. 2 and 3, the electrolysis cell E shown, and which is representative of the construction and operation of each of the cells E E inc., comprises a steel tank 42, lined with thermal insulating refractory brick 43. The top of the cell is made up of a number of precast shapes, as at 44, which rest on the main cell structure (not shown), and which support a series of spaced pairs of cathodes, as at 45, 46, and interposed anodes, as at 47. Interposed between each anode 47 and its associated pair of cathodes 45, 46, are a pair of semiwalls, as at 48, 49, for directing the chlorine and molten magnesium metal as freed from the molten salt bath 50, to the anodes and cathodes, respectively. The molten magnesium rises to the surface of the fused salt bath and floats thereon in the cathode compartments, as at 51, from whence it is continuously discharged as explained below. The chlorine gas rises into the spaces, as at 52, between the anodes and the semi-walls which dip into the bath as shown, and is drawn-off, through outlet ports, as at 53, provided in the sidewalls of the precast shapes in which the anodes are mounted. The outlet ports 53 are connected to the conduit C FIG. 1. The anodes 47 are made of graphite blocks, while the cathodes 45, 46, are steel casting, shaped to provide large parallel surfaces on opposite sides, respectively, of the interposed anode.

Referring to FIG. 3, the steel tank 42 and brick lining 43, are cut off along the tank sidewall, as at 55, 56, to

permit the molten magnesium metal 51 and a portion of the fused salt 50 to overflow, as at 55, through the outlet duct D and over a brickwork weir 57 mounted therein as shown. It will be observed from FIG. 3, that the top of the weir 57 is disposed above the lower edges of the semi-walls, the latter as indicated at 58, in order not to interfere with the function ofthe semi-walls in directing the chlorine gas and molten magnesium metal as formed, to the cell anodes and cathodes, respectively. During extended operation, as the semi-walls fail and break away in part along their lower edges, the level of overflow over the weir 57 may be compensatively raised by the addition of bricks thereto.

The electrolyte salt bath 50 of the cell E is preferably of the composition described in my copending application Ser. No. 448,852, filed Apr. 21, 1965, now abandoned, and consisting of about 525% magnesium chloride, 5- 55% lithium chloride, and the balance chlorides of one or more of calcium, potassium, and sodium, preferably in minimum amounts of about 5% each by weight of the total. A preferred bath is that consisting of about 50% LiCl, 5% NaCl, 10% KCl, 20% 02101 and 15% MgCI This bath is characterized by a density exceeding that of the molten magnesium, such that the magnesium metal as formed will rise to the surface of the bath and float thereon while the sludge being of greater density than the bath will sink to the bottom of the cell for automatic separation of the two. Also this bath by reason of its high lithium chloride content has much higher electrical conductivity and hence much lower ohmic resistance per unit volume than has heretofore been employed for the electrolytic production of magnesium metal in a bath of the density aforesaid insofar as I am aware.

Reference will now be had to FIGS. 4 and 5, wherein like elements are similarly designated as in FIGS. 1-3, inc. In this embodiment for practicing the invention on a commercial scale, the melt cell 15, comprises a reinforced outer steel housing 64, lined with refractory brick as at 65, the top of the chamber housing being removed to show the interior construction, and one of the resistance heating electrodes for converting the magnesium chloride to the molten state being shown at 18.

A pump 67, pumps the molten MgCl into the upper end of the enclosed feed launder 25, extending from the top of the melt cell 15 thence downwardly into a combined charging and refining cell unit 69, the top of cell unit 69 being also removed to show the interior construc= tion. The cell unit 69 is divided into two compartments by a refractory brick semi-wall 70, which divides it into the charging cell 26 on the left of the partition, and the refining cell 31 on the right. The refining cell is provided With a semi-wall 36, as in FIG. 1. Resistance heating electrodes, A.C. energized, are shown at 17, 18. A pump 77 pumps the molten magnesium chloride from the charging cell 26, upwardly over pipeline 32, and into the upper end of the feed launder 33, continuation of which is shown at 33 in FIG. 4. The overflow launder from the electrolysis cells is shown at 34 in FIG. 3, as a continuation of the like-numbered component of FIG. 4. As shown in FIG. 3, this overflow launder feeds into the refining cell 31, on the right-hand side of the semi-wall 36 as in FIG. 1. The molten magnesium metal is pumped from the refining cell, by means of pump 78, and delivered over a discharge line 79.

Referring to FIG. 5, the feed and overflow launders 33, 34, are of generally similar construction so that only one need be described in detail. Thus the feed launder 33, comprises an outer sheet metal trough-like casing 82, and an inner refractory brick lining 83, between which and the casing is interposed, a layer of thermal insulating material, such as glass Wool, as at 84, The resulting troughlike construction is covered by means of a series of removable thermal insulating blocks, as at 85, 86, of shallow T shaped cross-section, to seat into the trough of the basal assembly. From the feed launder 33, the cell launders as at P 1, P extend downwardly to the associated cells E 1, E

Electrolysis cell E 1 is shown with the cover and top assembly in place, while for cell E these components are removed. Referring to cell E 1, the cathode electrodes are shown as at 45, 46, and the interposed anode electrodes, as at 47. The cell overflow conduits D 1, D extend from the cells E -l, E respectively, to the overflow launder 34 as shown.

Referring to cell E the semi-Walls are shown at 48, 49, seated at opposite ends in slots of brickwork, as at 95, 96. The molten magnesium metal produced in the cell flows outwardly between slots as at 97, of contiguous brickwork supports, 96, 98, and thence over the Weir 57, in conduit D and thence into the overflow launder 34. The operation of the FIGS. 4 and 5 embodiment is the same as that above described with reference to FIGS. 1-3, inc.

The circulating cell melt system of the invention has the following improved characteristics as compared to prior art practices: All cells operate on the same melt composition. Individual cells operate at constant cell levels. Little if any variation in temperature occurs between cells since the melt is continuously blended. Little if any operating work is done on the cells. Magnesium is removed from the cells as formed, so that the chance of burning or recombination is minimized. Since little if any operating work is done on the cells, the cathode compartments can be sealed. Magnesium is removed from the electrolysis cells, as it is formed, and is conveyed thence to the refining cell in a closed conduit while being washed with cell melt, and is settled in the refining cell under a flux cover with good temperature regulation.

\Vhat is claimed is:

1. The method for continuous production of magnesium metal and chlorine gas from magnesium chloride, which comprises: progressively feeding substantially anhydrous magnesium chloride salt into a melt cell and fusing therein to a molten state, discharging said molten salt into a charging cell for admixture with a molten metal chloride salt containing a salt of at least one other metal, feeding the molten salt admixture from said charging cell thence into a feeder manifold and thence distributively into a series of electrolysis cells having overflow outlets for establishing and maintaining molten baths of said salt admixture therein, electrolyzing said magnesium chloride constituent therein into molten magnesium metal and chlorine gas, withdrawing said gas as formed from said cells into a collecting conduit, and separately withdrawing as overflow from said cells and into a collector manifold, said molten metal as formed together with a portion of the molten salt of each said cell, delivering the contents of said collector manifold into a refining cell and separating said molten metal from said fused salt therein, and recycling the magnesium chloride depleted molten salt from said refining cell into said charging cell for admixture with additional molten magnesium chloride salt supplied from said melt cell.

2. The method according to claim 1 wherein the salt in each of said melting, charging and refining cells is heated without decomposition by passage of an alternating current therethrough.

3. The method according to claim 1 wherein oxidic compounds in the salt charged into said melting cell are decomposed and the oxygen content eliminated in gaseous state by passage through said salt of direct current between spaced carbon electrodes contacting said salt.

4. The method according to claim 1 wherein the molten salt admixture fed to said electrolysis cells has a density exceeding that of molten magnesium metal, for floating the released molten magnesium metal thereon.

5. The method according to claim 1 wherein said molten salt admixture is gravity-fed from said feeder manifold into said electrolysis cells and is gravity-fed from said electrolysis cells into said collector manifold.

6. The method according to claim 1 wherein the molten salt admixture fed to said electrolysis cells consists of about 5-25% of magnesium chloride, 555% of lithium chloride by weight of the total, the balance being chloride salts of one or more of calcium, potassium and sodium, and wherein said molten salt admixture has a density exceeding that of molten magnesium metal.

7. The method according to claim 1 wherein said molten salt admixture fed to said electrolysis cells consists substantially of about lithium chloride, 5% sodium chloride, 10% potassium chloride, 20% calcium chloride, and 15% magnesium chloride, each by weight of the total.

References Cited UNITED STATES PATENTS 1,951,494 3/1934 Staib et a1. 204 2,162,942 6/1939 De Rohden 204-70 2,375,009 5/1945 Lepsoe et al. 204-70 3,312,607 4/1967 Goodenough et a1. 204-70 ROBERT K. MIHALEK, Primary Examiner.

DONALD R. VALENTINE, Assistant Examiner.

US. Cl. X.R. 204128, 246 

1. THE METHOD FOR CONTINUOUS PRODUCTION OF MAGNESIUM METAL AND CHLORINE GAS FROM MAGNESIUM CHLORIDE, WHICH COMPRISES: PROGRESSIVELY FEEDING SUBSTANTIALLY ANHYDROUS MAGNESIUM CHLORIDE SALT INTO A MELT CELL AND FUSING THEREIN TO A MOLTEN STATE, DISCHARGING SAID MOLTEN SALT INTO A CHARGING CELL FOR ADMIXTURE WITH A MOLTEN METAL CHLORIDE SALT CONTAINING A SALT OF AT LEAST ONE OTHER METAL, FEEDING THE MOLTEN SALT ADMIXTURE FROM SAID CHARGING CELL THENCE INTO A FEEDER MANIFOLD AND THENCE DISTRIBUTIVELY INTO A SERIES OF ELECTROLYSIS CELLS HAVING OVERFLOW OUTLETS FOR ESTABLISHING AND MAINTAINING MOLTEN BATHS OF SAID SALT ADMIXTURE THEREIN, ELECTROLYZING SAID MAGNESIUM CHLORIDE CONSTITUENT THEREIN INTO MOLTEN MAGNESIUM METAL AND CHLORINE GAS, WITHDRAWING SAID GAS AS FORMED FROM SAID CELLS INTO A COLLECTING CONDUIT, AND SEPARATELY WITHDRAWING AS OVERFLOW FROM SAID CELLS AND INTO A COLLECTOR MANIFOLD, SAID MOLTEN METAL AS FORMED TOGETHER WITH A PORTION OF THE MOLTEN SALT OF EACH SAID CELL, DELIVERING THE CONTENTS OF SAID COLLECTOR MANIFOLD INTO A REFINING CELL AND SEPARATING SAID MOLTEN METAL FROM SAID FUSED SALT THEREIN, AND RECYCLING THE MAGNESIUM CHLORIDE DEPLETED MOLTEN SALT FROM SAID REFINING CELL INTO SAID CHARGING CELL FOR ADMIXTURE WITH ADDITIONAL MOLTEN MAGANESIUM CHLORIDE SALT SUPPLIED FROM SAID METAL CELL. 